Apparatus for controlling the pressure in a well



g Q} EL in vnuuu nu ma num; QWWH fi-Uh Nbv. 11,1969 R. JONES ET AL3,477,526

APPARATUS FOR CONTROLLING THE PRESSURE IN A WELL Filed June 7, 1967 4Sheets-Sheet 1 k WW S, v Q

a; w m /=3 3 NR N N Ben/0x2 F. 300 74 Mar w R. (/0060 l I INVENTORJ & aMW BY 6M Arm/Mfr;

NOV. 11, 1969 JONES ET AL. 3,477,526

APPARATUS FOR CONTROLLING THE PRESSURE IN A WELL Filed June '7, 1967 4Sheets-Sheet 2 9,7 Ber; /0/7 E Baz/yfi Mow/v0 A? c/B/rar I I INVENTORJBY 4%;

W A rro/r/ws m Nov. 11, 1969 M. R. JONES IETAL 3,477,526

APPARATUS FOR CONTROLLING THE PRESSURE IN A WELL Filed June 7. 1967 4Sheets-Sheet 3 M 477mm; Ys

Nov. 11,1969 M. R JONES ETAL 3,477,526

I APPARATUS FOR CONTROLLING THE PRESSURE IN A WELL Filed June '7, 1967 4Sheets-Sheet 4 59/; I a/7 F. Ban 40% Mar u? A. M006;

INVENTORS' ATTORNEY! United States Patent 3,477,526 APPARATUS FORCONTROLLING THE YPRESSURE IN A WELL Marvin R. Jones and Benton F. Baugh,Houston, Tex., assignors to Cameron Iron Works, Inc., Houston, Tex.Filed} June 7, 1967, Ser. No. 645,566 Int. Cl. E2lb 3/12, 21/04, 7/00US. Cl. 175-25 ABSTRACT OF THE DISCLOSURE Control apparatus for use indrilling a well wherein drilling fluid is circulated through a drillstring extending into a 'wellbore and the annulus therebetween exitingwhen the well is shut-in through an outlet regulated by a choke. Thecontrol apparatus maintains the pressure differential between the bottomhole pressure of the drilling fluid and the formationfluid at apredetermined value. To accomplish this, the choke is provided withsignal-responsive operating means which moves a choke member toward andaway from a maximum flow-restricting position in accordance with signalsfrom the apparatus. Sensing devices at the inlet and outlet of the wellmeasure the inlet and outlet circulating pressures and, when there is nocirculation, the static pressure. A sensing device located at the inletis responsive to the density and circulation rate of the drilling fluid.Other parameters of the well are also determined. Signals correspondingto the parameters are fed into a console having a computer whichproduces from the input signals a control signal which is effective tourge the choke member toward maximum flow-restricting position. A signalrepresenting the outlet pressure is effective to urge the choke memberaway from the maximum flow-restricting position. The control signal andthe outlet pressure'signal cooperate to move the choke member to anequilibrium position at which the pressure differential is substantiallyequal to the predetermined value.

30 Claims This invention relates to the control of the pressure of fluidwithin a wellbore having a drill string extending into the bore. In oneof its aspects, it relates to such control upon entry of formation fluidinto the drilling mud within the annulus. In another of its aspects, itrelates to such control as the well is drilled under pressure. Moreparticularly, i t relates to improvements in apparatus and methods bywhich the well is controlled by means of a back pressureiimposed on theannulus at the head of .the well. In another of its aspects, it rel-atesto novel equipment especially well-suited for such use.

It has beeni customary to provide a choke in a manifold connecting withthe annulus beneath a blowout preventer closed about the drill string inorder to establish and maintain a back pressure on fluid divertedthrough the choke which, ''together with the hydrostatic pressure of themud, is sufficient to contain the pressure of fluids within formationspenetrated by the wellbore--i.e., preventsthem from flowing into thewellbore. In the case of a kick, the choke must continue to contain theformation fluid as heavier mud is circulated down the drill string andup the annulus to kill the well. The choke is prefer-ably adjustable sothat, in controlling the 'well pressure, an attempt may be made to avoidestablishing excessive back pressure which might cause the drill stringto stick, or damage a formation, the well casing, or the wellheadequipment.

In the use of one such system, when a kick is encountered, the preventerand choke manifold are closed, the mud pumps are stopped and shut-inpressure is observed at the manifold upstream of the choke. The pump isthen started slowly as the choke is gradually opened to maintain a backpressure at a level slightly above the observed shut-in pressure. Whenthe desiredcirculating rate is reached, it is held constant and thechoke is continuously adjusted to maintain the pressure in a standpipeconnected to the upper end of the drill string at the level it hasreached at such circulating rate. The constant pressure in the standpipeis maintained until the kick is circulated out of the annulus.

The mud weight increase necessary to contain the formation fluid iscalculated and the heavier mud is pumped into the drill string with thechoke being adjusted to maintain the annulus back pressure constant.When the heavier mud reaches bottom, the user begins to adjust the chokein order to again maintain the drill string pressure constant as suchmud circulates up throtilgh the annulus. Thus, in effect, the usermaintains a constant bottom hole pressure by controlling the pressure inthat portion of the 'well where the average density of the fluid in itis known more closely.

Our prior application Ser. No. 606,312 filed Dec. 30, 1966, now PatentNo. 3,429,385 discloses a system of this general type which not only isessentially automatic in that it does not require manual adjustments ofthe choke, but also enables the practice of a variety of methods in thatit does not require that the circulating rate be maintained constant.

With such a system signals are produced and delivered to the choke toautomatically regulate it so that a constant bottom hole pressure ismaintained. In the preferred embodiment thereof, a signal representingthe inlet pressure is applied to one side of the operator to move thechoke member away from maximum flow-restricting position and anothersignal, representing the ciilculating pressure loss in the drill stringplus the static pressure and a selected pressure which represents thepressure differential, is automatically computed and applied'to theother side of the operator to move the choke member toward maximumflow-restricting position. These two signals cooperate to move the chokemember to an equilibrium position whereby the deviation of the pressuredifferential from the predetermined value thereof is maintainedsubstantially at zero and the pressure at the bottom of the drill stringconstant.

While such a system will control the pressure differential, it has beenfound that gas in the wellbore increases the time required for apressure change at the choke manifold to travel through the wellbore andbe sensed at the upper end of the drill string. This lag tends to causeun desirable hunting or overcontrol ofthe outlet pressure and, as aconsequence, of. the inlet pressure. Also, the choke is slow to respondto sudden changes in the pressure of drilling fiuid at the outlet suchas arise when the material constituents of such fluid suddenly change.

It is an object of the present invention to provide improved apparatusfor maintaining the bottom hole pressure constant in which overcontroldue to lags in sensing pressure changes is substantially eliminated.

It is a further object to provide improved apparatus for maintaining thebottom hole pressure constant which adjusts to changes from gas toliquid flow through the choke quickly so as to avoid pressurefluctuations of significant magnitude or duration within the wellbore.

It is a further object to provide improved apparatus for maintaining thebottom hole pressure constant which utilizes an improved pneumaticcomputing circuit.

It is a still further object to provide methods by which these objectsare attained.

These and other objects are accomplished by apparatus and methods of thepresent invention wherein, in preferred embodiments, the well isprovided with a choke which regulates the outflow of drilling fluid whenthe well is shut-in. The choke is provided with a choke memher which ispositioned in response to a control signal in order to increase ordecrease flow restriction. The control signal may be opposed or biasedby another signal or other means. For example, this bias may be a signalrepresenting the outlet pressure, that is, it is a mathematical functionthereof, preferably linear. The bias signal and a control signal formedby combining signals representing the inlet pressure, the staticpressure, the predetermined pressure differential and the circulatingpressure loss in the drill string in accordance with a mathematicalfunction are applied to an operat ing means for the choke to urge thechoke member toward maximum flow-restricting position when the controlsignal is greater than the bias signal and away from maximumflow-restricting position when the bias signal is greater than thecontrol signal. The control signal increases or decreases when thedeviation by which the pressure differential exceeds the predeterminedvalue thereof is respectively less or greater than zero. As the controlsignal increases or decreases relative to the bias signal, the chokemember approaches an equilibrium position effecting an outlet pressureand thereby an inlet pressure at which the pressure differential issubstantially equal to the predetermined value. By continuous regulationof the choke in this manner, the pressure differential, and consequentlythe bottom hole pressure, is maintained substantially constant.

It has been found that by using the outlet signal as the bias the chokewill immediately respond to sudden changes in pressure of the drillingfluid at the outlet such as arise when the material constituents of thedrilling fluid suddenly change. It has also been found that by includingin the means for producing the control signal a. means for dampingchanges thereof during its transmission to the operating meansover-control and hunting are materially reduced.

It is to be understood that by bottom hole pressure is meant thepressure at the bottom of the drill string regardless of its position inthe wellbore, and that the formation pressure" when the drill string isnot on bottom is adjusted to the actual position of the bottom of thedrill string by reducing it by the amount of pressure due to the head offluid between the formation and the bottom of the drill string.

It is also to be understood that, while in the preferred embodiment theinlet for the drilling fluid is through a standpipe connected to thedrill string and the outlet is through a manifold connected below theblowout preventer, the circulation may be reversed with the inlet beingthrough the manifold and the outlet through a choke connected with thedrill string.

In the drawings, wherein like reference characters are used to designatelike parts,

FIG. 1 is a diagrammatic illustration of an apparatus constructed inaccordance with the present invention and installed upon a typical wellfor controlling same;

FIG. 2 is an enlarged perspective view of the console in FIG. 1 as seenfrom one corner thereof so as to illustrate its control panel;

FIG. 3 is a diagrammatic illustration of a preferred pneumatic-hydraulicembodiment of the apparatus;

FIG. 4 is a schematic of the apparatus shown in FIG. 3;

FIGS. 5 and 6 are schematics of alternate feedbacks to the computingrelay;

FIG. 7 is a schematic of alternate means for operating the choke fromthe signals developed by the apparatus; and

FIG. 8 is a schematic of alternate means for develop ing the outputsignal.

With reference now to the details of the abovedescribed drawings, thedrilling control apparatus illustrated in FIG. 1 is installed upon awell including a casing lining a portion of a wellbore 21 and a casinghead 22 connected to its upper end. A blowout preventer 23 connectedabove the casing head 22 has a bore 24 therethrough and reciprocablerams 25 mounted therein for closing the bore and shutting the well in.

A drilling bit 26 at the lower end of drill string 27 is rotated bysuitable well-known apparatus. A standpipe 28 is connected to the upperend of the drill string and drilling mud is circulated through thestandpipe, downwardly through the drill and bit, and upwardly withinannulus 29 between the drill string and the uncased and cased portionsof wellbore 21.

A side outlet 30 connects with the bore of the casing head beneath thepreventer. Upon closing the blowout preventer rams 25 about the drillstring, fluid within annulus 29 is diverted into outlet 30. Manifoldingin the form of a cross 31 connecting with the outlet 30 pro vides astraight run from the outlet and upper and lower wings to which chokes32 are connected. Preferably, at least one of these chokes isconstructed as disclosed in said prior application. The straight run andthe upper and lower wings of the cross are provided with valves, wherebyoutlet 30 may be closed or the fiow may be diverted through the straightrun or either choke 32.

Choke 32 includes a flow-restricting member 33 which is urged toward andaway from maximum flow-restricting position (i.e., closed or opened) byoperating means in response to signals through lines 35 and 36 fromconsole 37, shown in FIGS. 1 and 2 and to be described later.

A sensor and transmitter 38 connected to standpipe 28 has line 39leading therefrom for transmitting a signal S, representing the inlet orstandpipe pressure to console 37, which pressure is indicated on gauge40.

A sensor and transmitter 41 connected to outlet 30, upstream from choke32, has line 42 leading therefrom for transmitting a signal Srepresenting the outlet or choke manifold pressure to console 37, whichpressure is indicated on gauge 43.

A sensor and transmitter 44 is connected to standpipe 28 for sensing theproduct V of the mud density and the square of the circulating rate (Vand for transmitting, through line 45 to console 37, a signal Vrepresenting such product, which is indicated on gauge 46. Sensor 44 maybe such as described in said prior application.

Console 37 shown on a larger scale in FIG. 2 contains computing andsignal-producing apparatus which will later be described in connectionwith particular embodiments of the invention. The console (I) computesand transmits signals to the choke member operator means, (2) providesfor optional outlet (choke manifold) pressure control and (3) computesand indicates on gauge 47 the amount of mud weight increase necessary tocontrol the well.

In performance of the first function of console 37, and as illustratedin the preferred embodiment of the invention, the outlet signal S istransmitted through line 42 to a relay of the operating means to cause arelated signal to be transmitted through line 35 to a reciprocableactuator 34 as a bias to urge choke member 33 in opening direction. Theconsole also computes a control signal S which is transmitted throughline 81 to the relay 65 to cause a related signal to be transmittedthrough line 36 to the reciprocable actuator to urge member 33 inclosing direction.

To this end, the computer in the console first produces a signalrepresenting the circulating pressure loss:

P1=KD(PV2) qwhere K=Calibration factor D=Length of drill string (depth)p=Mud density V=Circulation rate.

For many purposes, equivalent results may be obtained by combining KDinto a single constant k. g

The V signal is received from line 45. Signals K and D representing thecalibration factor and depth are produced by adjusting knobs 48 and 49,respectively, with the value thereof indicated on gauges 50 and 51. Bymultiplication, the computer produces a signal 8, representing the.circulating pressure loss (KD V and indicates same on gauge 52.

A signal 8,, representing the shut-in static inlet pressure, adjusted byadding the predetermined value of the pressure differential betweenbottom-hole and formation pressures, positive or negative, is produced,after observation of said static pressure on gauge 40, by adjusting knob53 and the value thereof is indicated on gauge 54. If desired,individual signals representing static pressure and pressuredifferentialmay be utilized.

The forementioned signals are combined by the computer in console 37 toproduce a control signal S which is a function of the deviation, whichis given by the equation:

d= 1- 1- as q- B) where P =Deviation=P P' P =Pressure differential P=Predetermined pressure differential P =Inlet pressure P =Circulatingpressure loss P =Adjusted static pressure=P +P' The control signal Sincreases or decreases when P is respectively negative or positive andcooperates with the bias whereby choke member 33 seeks an equilibriumposition at which the deviation is zero, and the pressure differentialhas the predetermined value.

The second function of console 37, optional choke manifold control, isaccomplished by providing a switch 59 having a lever 59a whereby theabove control signal S may be replaced by the adjusted static pressuresignal S to place the operating means under control of the opposingoutlet pressure signal S in line 42.

The third function of console 37, computation of required mud Weightincrease, is accomplished by computer means in the console which combinethe static pressure and depth signals to indicate on gauge 47 suchrequired increase. As is well known, mud weight increase is proportionalto the quotient of the static pressure and the depth of the well.

A preferred embodiment of the apparatus in which the aforementionedsignals are pneumatic is shown in FIG. 3 and described therefrom. FIG. 4may be referred to for clarification. It is understood that such signalsmay also be made hydraulic, electric,- or combinations thereof byreplacing the elements described with equivalent elements well known tothose skilled in the art.

A source of pneumatic pressure (not, shown) supplies air through line60, dryer 61 and filter 62. Line 60 branches into lines 60a and 60bthrough pressure regulators 63 and 64 to provide air at two pressures(e.g. 2'0 and 40 p.s.i.) for operation of the transmitters and thepneumatic devices in the console.

Transmitter 38 senses the inlet pressure and transmits a pneumaticsignal 8, representing same through line 29 to gauge 40 and to computingrelay 57. Transmitter 41 senses the outlet pressure and transmits apneumatic signal S representing same throughline 42 to gauge 43 and tohydraulic relay 65. Transmitter 44 senses pV and transmits a pneumaticsignal representing same through line 45 to gauge 46 and to the sectionof the computer designed to produce a pneumatic signal S representingthe circulating pressure loss, now to be described.

Said section of the computer also receives calibration factor signal Kand depth signal D representing K and D in Equation A. Knob 48 adjustsregulator 48a to produce pneumatic signal K indicated on gauge 50. K maybe determined empirically or by well-known formulas.

Knob 49 adjusts regulator 49a to produce pneumatic sig-= nal D indicatedon gauge 51. The K and D signals are transmitted through lines 66 and 67to multiplying means 55 Which produces signal KD. The KD and p! signalsare transmitted through lines 68 and 45 to multiplying means 56 toprovide the circulating pressure loss signal 1 in accordance with Eq. A.This signal is transmitted through line 69, computing relay 70 (wherethe signal is doubled), line 71, amplifier 72 (where the signal isquadrupled) and line 73 to gauge 52 and to computing relay 57. Thesignal in line 69 is weak and is therefore boosted by a factor of eight.to its value in line 73.

The; multiplying means 55 and 56 may be Force Bridges such as thosemanufactured by Sorteberg Controls Company, South Norwalk, Conn., andthe computing relays 57 and 70 may be Nullmatic M/F Relays- Model 68,manufactured by Moore Products Co., Spring House, Pa.

A third signal fed to computing relay 57 represents adjusted staticpressure. With the well shut-in, the inlet pressure of gauge 40 is readto determine the static pressure. To this is added the predeterminedvalue of the differential between bottom hole and formation pressures.This adjusted static pressure is set on gauge 54 by adjusting pressureregulator 53a and knob 53. The resulting signal S is transmitted tocomputing relay 57 through line 75. One other signal processed by relay57 is the feed-back (indicated at S, in FIG. 4) of control signal 8,,the purpose and function of which will be subsequently described.

Thus, computing relay 57 receives signals 8,, S 8, and S,,. It combinesthese signals to produce an output signal S where If 8,, is a signalpressure representing the deviation Pd, then from Eq. B d i' l as andtherefore c"" co d Output signal S is transmitted through line 76,needle valve (or lag element) 74, and line 77 to deliver the controlsignal S to switch 59 which may be a three-way valve. A branch line 80from line 77 is connected to relay 57 for the feed-back of S Underdynamic conditions, there is a pressure drop across needle valve 74depending on the direction of flow. As seen from Eq. C, when thedeviation is positive, S is greater than S and flow of signal air istoward the relay 57, i.e. the relay is venting S and S is being reduced.When the deviation is negative, flow is from relay 57 and S is being increased.

Thus, feed-back of signal S results in a controlled damping acrossneedle valve 74, in that the system tries to maintain a pressure dropacross the valve equal to 8, This is particularly useful for low valuesof S,,.

From valve 59, the control signal S is transmitted through line 78,accumulator 79 and line 81 to hydraulic relay 65, which, in thepreferred embodiment, comprises a part of the means for operating thechoke member. It opposes the outlet signal S transmitted to the relaythrough line 42 as above described. Accumulator 79 serves as further lagmeans and cushions the control signal. Accordingly, the feed-back of thecontrol signal S to the computing relay 57, plus the damping action ofthe needle valve 74 and the cushioning action of the accumulator 79minimizes overcontrol and hunting.

Means are provided whereby the user may optionally supply thesubstantially constant static pressure signal S to the hydraulic relay65 in place of the control signal 8,. Thus, three-way valve 59 hasanother inlet to receive signal S through line 75. By moving lever 59aup or down, the user may put the choke on control signal control or onoutlet pressure control. In terms commonly used in the art, this is theequivalent of putting the choke on standpipe or choke manifold control,respectively.

Hydraulic relay 65 may be a Four-Way Hydraulic Valve, Model 79,manufactured by aforesaid Moore Products Co., modified to receive twoopposing pneumatic signals. It receives liquid under pressure throughline 82 and exhausts liquid through line 83. It delivers liquid to oneside or the other of actuator 34, exhausting from the opposite side, tomove the flow-restricting member 33 of choke 32 toward or away frommaximum flow restricting position as the signal in line 81, whether S orS becomes greater or less than the outlet signal S in line 42.

Means are provided for opening the choke manually. Thus, air supply line60a has a branch 84 through valve 85 to line 42. On opening valve 85,full supply pressure is transmitted through line 42 to relay 65. Suchpressure will cause relay 65 to deliver fluid to line 35 thereby movingflow-restricting member 33 in an opening direc tion.

Means are provided for indicating on gauge 47 on console 37 the amountof increase in mud density required to control the increase in formationpressure. Such increase is given by the known formula:

where P =static pressure D :depth The static pressure signal line 75 isbrought to com puting relay 86, where the signal is adjusted to asuitable range, and thence through line 87 to Force Bridge 58. Thesignal D representing the depth is transmitted to Force Bridge 58through line 67. The bridge computes an output signal in accordance withEq. D which signal is transmitted through line 88 to gauge 47.

In preparing to control the well with this apparatus and while drillingwith the well open, the user may, from time to time, adjust knob 49 tocorrect depth signal D. The V signal is automatically sensed and enteredinto the computer continuously during circulation of the mud. Thus,after adjusting for depth, the user need only compare the circulatingpressure loss on dial 52 with inlet pressure on dial 40. If they are notin agreement, the user adjusts the calibration knob 48 to bring thecirculating pressure loss into agreement with the inlet pressure. Aspreviously described, change in well depth may be disregarded.

The computed circulating pressure loss takes into account the loss inthe annulus as well as the loss in the drill string. While thecirculating pressure loss in the annulus is relatively small and may beignored, it may be preferabl e to reduce the circulating pressure lossby approximately ten percent. This may be accomplished by adjustingcalibration knob 48 until the circulating pressure loss is ninetypercent of the inlet pressure, and thus approximately the circulatingpressure loss in the drill string.

The user further prepares for the kick" by moving switch lever 59a tostandpipe control. When a kick is encountered, the user picks the drillbit up off the bottom of the hole, shuts down the mud pumps, and closesthe blow-out preventer rams 25 about the drill string 26 and opens thevalve to the choke manifold. After a short wait, the inlet pressure ondial 40 and the outlet pressure on dial 43 are read and recorded. Withno circulation these are static pressures. The user then sets dial 54 bymeans of knob 53 to indicate the static inlet pressure which was on dial40 plus the predetermined pressure differential and starts the mudpumps. The circulating pressure loss signal S the inlet pressure signalS and the adjusted static pressure signal S are all transmitted tocomputing relay 57 where together with the control signal S they areprocessed to produce output signal S With the switch 59 on standpipecontrol the control signal S is transmitted through line 78 andaccumulator 79 to one side of relay 65. At the same time the outletpressure signal S is transmitted by line 42 to the otehr side of relay65. If control signal S is larger than outlet signal S relay willtransmit pressure to actuator 34 to urge the choke member in a closingdirection. If outlet signal S is larger, the choke member will be urgedin an opening direction. The operating means thus positions the chokemember so as to maintain outlet pressure at the value represented by thecontrol signal which, in the equilibrium position, effects an outletpres sure and thereby an inlet pressure at which the pressuredifferential at the bottom of the drill string is substantially equal tothe predetermined value. Thus, the well is controlled during thecirculation out of the kick by maintaining the pressure differential atthe bottom of the drill string constant.

If a piece of shale enters the choke and blocks the opening between theflow-restricting member and the seat, the outlet pressure will increase.Therefore, the outlet signal S will also immediately increase which willcause an imbalance at relay 65. However, there will be no increase ininlet pressure at this time and, therefore, there will be no change inthe control signal S With the out let signal S being greater than thecontrol signal S the shuttle in hydraulic relay means 65 will movecausing hydraulic supply to be provided to line 35 to open the choke. Assoon as the shale goes through the choke, the outlet pressure will againdrop and the apparatus will come back into equilibrium at the positionprior to the blockage. It has been found that generally the increase inoutlet pressure will be of such a temporary nature that its effect willnot be transmitted to the inlet.

If, during circulation of a kick out of the well, there is anydisturbance in well pressure values such disturbance will be reflectedin the inlet circulating pressure which will in turn produce a signalchange. If the inlet pressure increases, the output signal fromcomputing relay 57 will be lowered to maintain a bias across lag element74 which is proportional to that produced by the inlet pressure signalminus the sum of the circulating pressure loss signal and the adjustedstatic pressure. Thus, a small change in the inlet pressure produces asmall bias across the lag element and establishes a period 'of time forthe required change in the control signal which is in the accumulator totake place. If, on the other hand, there is a change in the circulatingpressure loss signal the same chain of events will take place.Therefore, in view of the lag element 74 there will always be a gradualchange in control signal S in response to fluctuations or changes ininlet pressure or circulating pressure loss. At the same time there willbe a rapid response to changes in outlet pressure.

When the kick has been circulated out of the wellbore switch 59 may bemoved to choke manifold control position, and heavier mud circulateddown through drill string 26 to the bottom of the hole. The added mudweight necessary in order to provide an adequate hydrostatic pressurewhen such mud has reached the bottom of the hole has, of course, beencomputed in the manner described and indicated on dial 47.

Movement of switch 59 to choke manifold control moves the valve memberto the inlet position receiving the adjusted static pressure signal STherefore, such signal is transmitted through line 75, switch 59 to line78 to accumulator 78 where it is transmitted to relay 65 to urge theflow-restricting member toward maximum fioW-resistricting position. Asin the case of. standpipe control, the outlet pressure signal 5 istransmitted by line 42 to relay 65 for urging the flow-restrictingmemher away from maximum flow-restricting position. Since the pressurein line 81 corresponds to the adjusted static pressure, this signal is aconstant so that the flow-restricting member of the choke automaticallyadjusts in such a manner that the opposing signal through line 42, whichcorresponds to the outlet pressure, remains constant. This continuesuntil the heavier .mud is pumped to the bottom of the hole.

At this time, the user turns the knob 53 to change the reading on staticpressure dial 54 to zero, and moves switch 59 to standpipe control.Since static pressure is zero, the signal which is transmitted tocomputing relay means 57 by means of line 75 is zero. Therefore, theoutput signal of relay 57 corresponds only to the inlet pressure minuscirculating pressure loss plus the control pressure. This signal, ofcourse, is opposed by a signal corresponding to the outlet pressuretransmitted through line 42 to the relay 65. The user begins to weighthe mud returns as soon as the outlet pressure reads zero. When theweight of the returns approach that of the heavier mud, the user checksthe hole to see if it runs over with the mud pump stopped. If it doesnot, he knows the well is killed.

In the event of a severe kick, or with expensive rig rates, the user maywant to start killing the well at the same time he starts to circulatethe kick out of the annulus. In doing so, he will reduce the amount ofpressure built up on the annulus, and also reduce the amount of time thedrill bit is inactive.

In this latter alternative method, he follows the same initial steps asin the other method above-described when he encounters a kick. That is,he picks the bit up off the bottom of the hole, he shuts the mud pumpsdown, and closes the preventer rams about the drill string. Furthermore,after the well has been shut-in for a short time, he determines thestatic pressure and sets the static pressure adjusted by the addition ofthe predetermined pressure differential. He further reads and recordsthe outlet and inlet pressures, moves switch 59 to standpipe controlposition, and resumes mud circulation.

In this method, the user immediately begins to circulate the mud intothe well at whatever rate he is able to mix the mud. He reads the mudweight increase on dial 47 and mixes his mud accordingly, knowing thatthis reading includes any overbalance he has added to the staticpressure. The user determines how long it takes the new mud to get tothe bit at the bottom of the drill string, and reduces the staticpressure gradually so that it reaches zero when the heavier mud arrivesat the hit.

As the heavier mud reaches the sensing device 44, the user observes thechange in circulating pressure loss reflected on dial 52, and reducesthe static pressure reading on dial 54 a corresponding amount bysuitable adjustment of the knob 53. He then continues to adjust thisknob in order to continuously reduce the static pressure, as adjusted,in proportion to the depth reached by the mud, until such pressurereading is zero. He also watches the outlet pressure dial 43 and when itreaches zero he knows that the well should be dead. He then begins toweigh mud returns, and when they are within a point or two of theheavier mud weight, he checks the hole to see if it will run over. If itdoes not, he knows that the well is dead.

Other methods may be advisable under these same or different conditions,and the use of such methods with this system are contemplated by thepresent invention. Also, of course, the user may use this system indrilling under pressure, in which case he merely follows thoseprocedures described in accordance with the first method during theinitial standpipe control of the well.

In the preferred embodiment the control signal S is fed back into thecomputing relay 57, as indicated at 8;; therefore the output signal S isa function of the amount of difference in the opposing signals and thereIS a -very gradual control signal response to changes in inlet pressure.However, if desired, the output signal S itself may be fed back into thecomputing relay 57, see S; in FIG. 6, in which case the output signalwill be a function of the amount of difference between the supplypotential and the value of the control signal. FIG. 5 shows anotheralternate means of feed-back in which case the outlet signal S isutilized, as indicated at 5;.

While the preferred embodiment shown in FIGS. 3 and 4 utilizes hydraulicfluid for operating the actuator, pneumatic pressure may also be used.This is shown in 7, where it can be seen that instead of feeding theoutlet signal S and control signal S into hydraulic relay valve 65, theoutlet signal S in line 42 and the control signal S in line 81 are fedinto amplifiers and 91. The amplifiers are, of course, connected topneumatic supply pressure. The outputs from the amplifiers are connectedto lines 35 and 36 for transmission of pneumatic pressure to theactuator 34.

-.Also, instead of having the circulating pressure loss signal S and theadjusted static pressure signal S fed directly into computing relay 57,these signals may be combined in computing relay 93 and the sum thereofthen fed into a computing relay 94. In such case, signals will besuitably increased in value by amplifiers, see FIG. 8. The feed-backsignal Sf shown in FIG. 4 may be S as in FIG. 4, S as in FIG. 5, or S asin FIG. 6. Other changes in circuitry in line with the basic premisethat a bias is applied on one side of the operating means and a controlsignal, which is a function of the inlet pressure, circulating pressureloss and adjusted static pressure, is applied on the others ide of theoperating means may be made. In all cases, the choke should tend toachieve an equilibrium position which will maintain a'predeterminedpressure differential between the bottom hole pressure and the formationpressure with the choke being responsive to changes in inlet pressure orcirculating pressure loss so that an equilibrium position will beestablished which will maintain the bottom hole pressure constant.

I The invention having been described, what is claimed is:

1. For use in drilling a well into an earth formation containing fluidunder pressure, wherein drilling fluid is c rculated through a drill'string extending into the wellbore and through the annulus therebetween,said string and annulus having upper ends, one of which is an inlet andthe other an outlet, there being a positive or negative pressuredifferential by which the bottom hole pressure of the drilling fluidexceeds the formation fluid pressure, the deviation, positive ornegative, by which isaiil pressure differential exceeds a predeterminedpressure differential being equal to a mathematical function of thedrilling fluid inlet pressure, the static pressure at the inlet when thewell is shut-in, the circulating pressure loss in the drill stirmg, andsaid predetermined pressure differential; appfiratus for maintainingsaid pressure differential at the predetermined value thereof,comprising a choke for regulating the outflow of drilling fluid, saidchoke having a flow-restricting member movable toward and away from aposition of maximum flow restriction, and operatmg means for so movingthe flow-restricting member, said operating means being responsive to acontrol signal to cause the flow-restricting member to move away frommaximum flow-restricting position and to a bias to cause theflow-restricting member to move toward flow-restricting position, abias, means for sensing the inlet pressure and the static pressure,means for adjusting the static pressure by adding the predeterminedpressure differential thereto, means for computing the circulatingpressure loss, means for producinginput signals representing said inletpressure, adjusted static pressure and circulating pressure loss, andmeans for combining said input signals in accordance with saidmathematical function and for producing therefrom a control signal whichincreases or decreases as said deviation becomes respectively negativeor positive, whereby the flow-restricting member approaches anequilibrium position at which the outlet pressure and hence the inletpressure are such as to effect a zero deviation with the pressuredifferential substantially equal to said predetermined value thereof.

2. The apparatus specified in claim 1 wherein the mathematical functioncontrolling the deviation is as follows:

P is the deviation,

P is the inlet pressure,

P is the static pressure,

P is the circulating pressure loss,

P' is the predetermined pressure differential, and P +P' is the adjustedstatic pressure.

3. The apparatus specified in claim 2 wherein the means for combiningthe signals is a computing relay means.

4. The apparatus specified in claim 3 wherein the means for producingthe control signal includes an accumulator between the relay means andthe operating means.

5. The apparatus specified in claim 3 wherein the means for producingthe control signal includes a needle valve.

6. The apparatus specified in claim 5 including means for feeding thecontrol signal to the computing relay means and bypassing the needlevalve so as to add it to the combination of said input signals.

7. Apparatus for use as part of a pressure control system for drilling awell, wherein a circulating medium flows through a drill string and thesystem includes a choke regulating the outflow of the circulatingmedium, the choke having a flow-restricting member movable toward andaway from a maximum flow-restricting position, said apparatus comprisinga console having a control panel, operating means for so moving theflow-restricting member, said operating means having first and secondsignal responsive sides, means in the console for automaticallyproducing a first fluid pressure signal to be applied to the first sideof the fluid pressure responsive operating means, said first signalrepresenting the outlet circulating pressure, means in the console forautomatically producing a second fluid pressure signal for selectiveapplication to the second side of the operating means, the second signalrepresenting a function of the inlet circulating pressure, a computedcirculating pressure loss, and the shut-in inlet static pressure plus apredetermined pressure differential, means in the console forautomatically producing a third fluid pressure signal for selectiveapplication to the second side of the operating means, said third signalrepresenting the shut-in inlet static pressure plus the predeterminedpressure differential, and means for so selecting the second or thirdsignal to be transmitted to the second side of the operating means, andmeans for transmitting said first signal and the selected second orthird signal to the first and second sides, respectively, of theoperating means.

8. Apparatus specified in claim 7 including a needle valve between themeans for producing the second signal and the selecting means.

9. Apparatus specified in claim 8 including an accumu= lator betw'en theselected means and the second side of the operating means.

10. Apparatus specified in claim 7 including a source of fluid pressure,means for connecting the source with the means for transmitting thefirst signal to the operat ing means, a valve in the connecting means,said valve being normally closed, the opening of said valve deliveringfluid pressure to the transmitting means to cause the choke to open.

11. Apparatus specified in claim 7 wherein the operat" ing meanscomprises an actuator for connection to the choke member, a source ofhydraulic fluid pressure, and a relay for controlling the flow of saidhydraulic fluid to and from said actuator in response to said signals.

12. Apparatus for use as part of a pressure control system for drillinga wel, wherein a circulating medium flows through a drill string and thesystem includes a choke having a flow-restricting member for regulatingthe outflow of the circulating medium, said apparatus comprising chokeoperating means, a source of energy, means to automatically modulatesaid source of energy to produce a first signal representing the outletpressure, means to automatically modulate said source of energy toproduce a second signal representing the circulating pressure Within theinlet end of the well, a computed circulating pressure loss within thewell, and the shut-in static pressure of the well when shut-in plus apredetermined pressure differential, and means for transmitting saidfirst and second signals to the operating means, said first signal beingapplied to operating means to cause the flow-restricting member to bebiased away from the maximum flowrestricting position and said secondsignal being applied to the operating means to automatically cause theflow-- restricting member to be moved toward maximum flowrestrictingposition when the sum of the computed circulating pressure loss and thestatic pressure plus the predetermined pressure differential is greaterthan the inlet circulating pressure, and to automatically cause theflow-restricting member to be moved away from maximum flow-restrictingposition when the inlet circulating pressure is greater than the sum ofthe computed circulat ing pressure loss and the static pressure plus thepredetermined pressure differential.

13. The apparatus specified in claim 12 wherein the source of energy ispneumatic fluid.

14. The apparatus specified in claim 13 wherein the means forautomatically computing the circulating pressure loss comprises means tomodulate the pressure of the pneumatic fluid to independently produce afirst signal representing the density of a circulating medium and thesquare of its circulating rate V a second signal representing the depthof the well (D), and a third signal representing a calibration factor(K), and means to com bine said first three signals to compute a signalrepresenting circulating pressure loss (P by the equation 15. For use indrilling a well into an earth formation containing fluid under pressure,wherein drilling fluid is circulated through a drill string extendinginto a wellbore and through the annulus therebetween, said string andannulus having upper ends, one of which is an inlet and the other anoutlet, there being a pressure differen" tial, positive or negative, bywhich the bottom hole pres sure of the drilling fluid exceeds theformation, fluid pressure and there being a deviation, positive ornegative, by which said pressure differential exceeds a predeterminedvalue thereof; apparatus for maintaining said pres sure differential atthe predetermined value thereof, comprising choke means for regulatingthe outlet fluid pressure to a bias signal and a control signal, meansfor sensing the drilling fluid outlet pressure and producing a biassignal corresponding to said sensed pressure, and means for producing acontrol signal which in cooperation with the bias signal causes thechoke means to increase or decrease the outlet fluid pressure when saiddeviation is respectively negative or positive, whereby the outletpressure approaches a value at which said deviation is zero.

16. The apparatus specified in claim 15, wherein the means for producingthe control signal includes a means for damping changes therein.

17. For use in drilling a well into an earth formation containing fluidunder pressure, wherein drilling fluid is circulated through a drillstring extending into a wellbore and through the annulus therebetween,said string and 13 annulus having upper ends, one of which is an inletand the other an outlet, there being a pressure differential, positiveor negative, by which the bottom hole pressure of the drilling fluidexceeds the formation fluid pressure, and there being a deviation,positive or negative, by which said pressure differential exceeds apredetermined value thereof; apparatus for maintaining said pressuredifferential at the predetermined value thereof, comprising achoke forregulating the outlet fluid pressure in response to a control signal anda bias, means providing a bias, and means for producing a control signalwhich cooperates with the bias to cause the choke to increase ordecrease the outlet pressure automatically in response to said deviationbeing respectively negative or positive, whereby the outlet pressureapproaches a value at which said deviation is zero, said last-mentionedmeans including means for damping changes in the control signal,

18. For use in drilling a well into an earth formation containing fluidunder pressure, wherein drilling fluid is circulated through a drillstring extending into the wellbore and through the annulus therebetween,said string and annulus having upper ends, one of which is an inlet andthe other an outlet, the pressure differential by;which the bottom holepressure of the drilling fluid exceeds the formation fluid pressurebeing equal to a mathematical function of the drilling fluid inletpressure, the inlet static pressure when the well is shut-in and thecirculating pressure loss in the drill string; apparatus for maintainingsaid pressure differential at a predetermined value, comprising a chokefor regulating the outflow of the drilling fluid, said choke having aflow-restricting member movable toward and away from a maximumflow-restricting position, and operating means for so moving theflowrestricting member in response to a bias signal and a controlsignal, means for sensing the drilling fluid outlet pressure andproducing a bias signal corresponding to said outlet pressure which iseffective to urge the flowrestricting member away from maximumflow-restricting position, means for sensing said inlet pressure andstatic pressure, means for computing circulating pressure loss, meansfor producing input signals corresponding to said sensed and computedpressures and a signal corresponding to said predetermined value of thepressure differential, and means combining said input signals inaccordance with said mathematical function to produce a control signalwhich increasesv or decreases when the function, and thus the pressuredifferential, is respectively less or. greater than said predeterminedvalue, and is effective-{to urge the flow-restricting member towardmaximum flow-restricting position, said control signal cooperating withsaid bias signal to move the flow-restricting member toward or away frommaximum flow-restricting position as the control signal respectivelyincreases or decreases relative to the bias signal, whereby theflow-restricting member approaches an equilibrium position effecting anoutlet pressure, and thereby an inlet pressure, at which the pressuredifferential is substantially equal to said predetermined value thereof.

19. Apparatus as in claim 18, including means for selectively renderingsaid operating means responsive to said first mentioned control signalor a second control signal equal to said signal corresponding to staticpressure.

20. The apparatus as in claim 18, wherein the mathematical function towhich the pressure differential is equal 15:

d=f( 1'- s 1) where P is the pressure differential,

P, is the inlet pressure,

P is the static pressure, and

P is the circulating pressure loss.

21. The apparatus as in claim 18, wherein the means for producing thecontrol signal includes a means for damping changes in the controlsignal.

22. For use in drilling a well into an earth formation containing fluidunder pressure, wherein drilling fluid is circulated through a drillstring extending into the wellbore and through the annulus therebetween,said string and annulus having upper ends, one of which is an inlet andthe other an outlet, the pressure differential by which the bottom holepressure of the drilling fluid exceeds the formation fluid pressurebeing equal to a mathematical function of the drilling fluid inletpressure, the inlet static pressure when the well is shut irr-,;and thecirculating pres sure loss in the drill string; apparatus formaintaining said pressure differential at a predetermined value,comprising a choke for regulating tlie outflow of the drilling fluid,said choke having a flow-restricting member movable toward and away froma maximum flow-restricting position, and operating means for so movingthe flowrestricting member in response Yto a bias signal and a controlsignal, means for producing a bias signal which is effective to urge theflow-restricting member away from maximum flow-restricting position,means for sensing said inlet pressure and static pressure, means forcomputing circulating pressure loss, means for producing input signalscorresponding to said sensed and computed pressures and a signalcorresponding to said predetermined value of the pressure differential,and means combining said input signals in accordance with saidmathematical function to produce a control signal which increases ordecreases when'the function, and thus the pressure differential, isrespectively less orgreater than said predetermined value, and iseffective to urge the flow-restricting member toward maximumflow-restricting position, said control signal cooperating with saidbiasf signal to move the flowrestricting member toward or away frommaximum flowrestricting position as the control signal respectively increases or decreases relative to :the bias signal, whereby theflow-restricting member approaches an equilibrium position effecting anoutlet pressure, and thereby an inlet pressure, at which the pressuredifferential is substantially equal to said predetermined value thereof,said last-men tioned means including means for damping changes in thecontrol signal. 1

23. The apparatus as in claim 22, wherein the means for producing thecontrol signal comprises a source of supply pressure, relay means fordelivering supply pressure to the operating means when the algebraic sumof the circulating pressure loss, plus the shut-in static pressure,;

24. The apparatus as in clairii 23, including means for combining andintroducing the; static pressure and the.

pressure differential into the reljiy as a single signal.

25. The apparatus as in claim 23, wherein the damping means includes anaccumulator between the relay means and the operating means.

26. The apparatus as in claim 23, wherein the damping means includes aneedle valve between the relay means and the operatingimeans.

27. The apparatus as in claim 26, wherein the means for producing thecontrol signal includes means for feeding the signal between the needlevalve and the operating means back into the relay means, so as to add itto said algebraic sum.

28. The apparatus as in claim 26', wherein the means for producing thecontrol signal includes means for feeding the signal between the relaymeans and the needle valve back into the relay means, so as to add it tosaid algebraic sum.

29. The apparatus as in claim 26, wherein the means for producing thecontrol signal includes means 'for feeding the bias signal into therelay means, so as to add it to said algebraic sum.

30. The apparatus as in claim 26, wherein the damping means alsoincludes an accumulator between the needle valve and the operatingmeans.

References Cited UNITED STATES PATENTS Re. 26,220 6/1967 Records 175-25X 2,786,652 3/1957 Wells 175-25 3,268,017 8/1966 Yarborough 17525 163,338,319 8/1967 Griffin 175-25 3,362,487 1/1968 Lindsey 175-383,372,761 3/1968 Van Gils 175-25 CHARLES E. OCONNELL, Primary ExaminerIAN A. CALVERT, Assistant Examiner US. Cl. X.R.,

